As an antenna device for which a MIMO (Multi-Input Multi-Output) technique of transmitting and receiving wireless signals of a plurality of channels at the same time by using a plurality of antennas is adopted, there exists a MIMO antenna device disclosed in Patent Document 1, for example.
The conventional MIMO antenna device disclosed in Patent Document 1 includes four antenna element groups located at equal intervals and a main body section. Each of the antenna element groups includes four antenna elements which have polarization directions different from each other. On the other hand, the main body section includes: a switch section connected to the antenna elements; a signal receiving section for receiving a reception signal via the switch section; an antenna control section for generating a control signal for the switch section; an antenna selection section for generating combinations of the antenna elements and to notify the antenna control section of information of the selected element; and an antenna determination section for specifying a particular combination of the antenna elements based on the reception signal received by the antenna elements included in the combinations generated by the antenna selection section, and to notify the antenna control section of information of the specified element.
This conventional MIMO antenna device having the above configuration is used for reducing correlation among the antenna elements and ensuring sufficient transmission capacity, by specifying a combination of the antenna elements so as to select one antenna element from each of the antenna element groups.
That is, in the conventional MIMO antenna device, a plurality of antenna elements are operated concurrently and then each of the antenna elements obtains as large amount of received electric power as possible, thereby increasing total transmission rate of a plurality of signal sequences to which MIMO demodulation has been performed. In order to achieve this, the MIMO antenna device disclosed in Patent Document 1 includes more antenna elements than channels of simultaneous communication performed by the MIMO antenna device, and among the antenna elements, antenna elements which are receiving signals having large intensities are selected so as to perform MIMO demodulation.
Selecting antenna elements as described above is effective especially in a case where, in a mobile communication, due to movement of a mobile station (a user) or temporal variation of an ambient environment, signal intensities of a main polarized wave and a cross polarized wave temporally vary or the arrival angles vary. Moreover, variation of polarization direction can be dealt with by using antenna elements having polarization characteristics different from each other, and the temporal variation can be resolved by performing control so as to switch among the antenna elements.
As described above, the MIMO antenna device disclosed in Patent Document 1 includes a plurality of antenna element groups each including a plurality of antenna elements, and, by using the switch section, selects a combination of the antenna elements having minimum correlation thereamong or selects a combination of the antenna elements having maximum transmission capacity, thereby enabling reduction of correlation among the antenna elements and enhancement of transmission capacity.
Next, with reference to Patent Documents 2 and 3, one example of mobile wireless apparatuses a part of which is used as an antenna will be described.
In a mobile wireless apparatus disclosed in Patent Document 2, a part of the housing conductor of the mobile wireless apparatus is caused to operate as a part of an antenna such that dedicated components for antennas are not needed, the number of pieces of components is reduced to decrease the manufacturing cost, and thinned size and reduced weight of the device are obtained. Moreover, using the housing as an antenna enables the size of an antenna to be increased, and then it is expected that the sensitivity of an antenna is enhanced. As described above, when the mobile wireless apparatus disclosed in Patent Document 2 is used for a mobile terminal which is desired to be miniaturized, the mobile wireless apparatus causes the housing conductor to operate as a part of an antenna, thereby improving the quality of wireless communication.
A mobile telephone disclosed in Patent Document 3 is used for reducing gain fluctuation depending on a state of a hand of a user. Patent Document 3 discloses a configuration that in a flip-type telephone 1, a shield box 14 in an upper housing 3 and an output terminal of a transmission circuit 15 in a lower housing 4 are connected to each other by a flexible cable 9 and the shield box 14 is used as an antenna (FIG. 3 in Patent Document 3). Thus, by using the shield box 14 as an antenna, gain fluctuation depending on a state of a hand of a user can be reduced.
Next, with reference to Patent Document 4, one example of a ¼ wavelength one-end-short-circuited patch antenna having a small ground plane will be described.
The ¼ wavelength one-end-short-circuited patch antenna disclosed in Patent Document 4 enables the size of a ground plane to be reduced, so that the bandwidth of the antenna can be increased and the size of a wireless apparatus can be reduced. The ¼ wavelength one-end-short-circuited patch antenna has a feature that, particularly in a miniaturized mobile wireless apparatus such as a pager, gain reduction due to approach of an electric circuit and a human body is suppressed owing to a thinned patch structure, and the size and the weight thereof are substantially reduced. That is, the antenna of the conventional art is suitable as an antenna for a miniaturized mobile wireless apparatus, and in mobile terminals which are desired to be miniaturized, it is expected that a miniaturized and thinned patch structure improves the quality of wireless communication.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2004-312381    [Patent Document 2] Japanese Laid-Open Patent Publication No. 2004-274730    [Patent Document 3] Japanese Patent Specification No. 3830773    [Patent Document 4] Japanese Examined Patent Publication No. 6-1848